1. Field of the Invention
The present disclosure relates to a non-volatile storage device, such as a Solid State Drive (SSD) or a semiconductor memory card, having rewritable non-volatile memory such as flash memory as a data storage medium, a non-volatile storage system including the non-volatile storage device, and a memory control method for non-volatile memory.
2. Description of the Related Art
A non-volatile storage device having a rewritable non-volatile memory such as flash memory is widely used as an SSD and a semiconductor memory card, for example. The non-volatile storage device is much in demand as non-volatile storage system combining the non-volatile storage device with AV equipment such as a personal computer and a digital camera.
The SSD and the semiconductor memory card include a flash memory to store data and a memory controller to control the flash memory. The memory controller is a device for controlling reading data from the flash memory and writing data to the flash memory, according to a read instruction of data and a write instruction of data from an access apparatus such as the personal computer or the digital camera. The memory controller typically performs controls such as defect management, wear leveling (leveling of the number of times of rewriting), and error correction processing by Error Correcting Code (ECC), to the flash memory. A typical type of the flash memory is a NAND type.
In recent years, a data retention period of the flash memory tends to be short due to semiconductor manufacturing process shrink and multi-level memory cells. A method disclosed in Patent Literature 1, for example, for this problem. This method is that a non-volatile storage device performs a refresh operation in accordance with a result of comparison between urgency of a level set for execution of refresh operation and a level of a data change state. This method allows the non-volatile storage device to prevent reduction in a processing speed and perform an appropriate refresh operation according to situations.